The MUSCULAR programme, revealed by the Washington Post in October 2013 based on Snowden documents, showed that the NSA and GCHQ had secretly tapped the unencrypted fibre-optic links connecting Google's and Yahoo's data centres to one another. Unlike PRISM, which used legal orders to compel disclosure, MUSCULAR accessed data in transit on private backbone infrastructure without the companies' knowledge. The disclosure prompted both companies to encrypt their internal links. MUSCULAR is confirmed by NSA slides published by the Washington Post.